1. Field of the Invention
The invention relates to a method of producing a lithium ion secondary battery and to a lithium ion secondary battery.
2. Description of Related Art
Lithium ion secondary batteries are receiving attention as power sources for mobile and portable devices and as power sources for, for example, electric automobiles and hybrid automobiles. Lithium ion secondary batteries are available in which a battery case houses a nonaqueous electrolyte solution and an electrode assembly having a positive electrode that contains a positive electrode active material (refer, for example, to Japanese Patent Application Publication No. 2004-31079 (JP 2004-31079 A)).
JP 2004-31079 A discloses a lithium ion secondary battery that is produced using a nonaqueous electrolyte solution that contains the compound with formula (1) below (lithium difluorophosphate; this compound is also referred to below as LiPO2F2).

After a lithium ion secondary battery that contains an LiPO2F2-added nonaqueous electrolyte solution has been assembled, a film deriving from the LiPO2F2 can be formed on the surface of the positive electrode active material by activating the battery (for example, performing the initial charge). The formation on the surface of the positive electrode active material of a film deriving from the LiPO2F2 can inhibit the formation of a film deriving from the electrolyte or solvent in the nonaqueous electrolyte solution. The film deriving from the LiPO2F2 has a lower resistance and is more stable than films deriving from the electrolyte or solvent in the nonaqueous electrolyte solution. An increase in the battery output can thus be achieved. In addition, the LiPO2F2-derived film formed on the surface of the positive electrode active material can inhibit degradation reactions (exothermic reactions) by the solvent in the nonaqueous electrolyte solution at the positive electrode and as a consequence can suppress heat generation by the battery (particularly heat generation during overcharging).
Thus, the goal of the addition of LiPO2F2 is to form an LiPO2F2-derived film on the surface of the positive electrode active material and thereby to raise the battery output and restrain the amount of heat generated during charging. In addition, the specifications for lithium ion secondary batteries are quite diverse, and the amount of nonaqueous electrolyte solution that is incorporated and the specific surface area of the positive electrode active material thus also vary among lithium ion secondary batteries. Given that the LiPO2F2-derived film is formed on the surface of the positive electrode active material, it can be contemplated that the amount of LiPO2F2 addition would preferably be determined in conformity with the specific surface area of the positive electrode active material.
On the other hand, in JP 2004-31079 A the amount of LiPO2F2 addition is defined by the weight % concentration or molar concentration in the nonaqueous electrolyte solution without considering the specific surface area of the positive electrode active material. However, when the amount of LiPO2F2 addition is defined in this manner, the amount of LiPO2F2 can then be too small or in excess in relation to the specific surface area of the positive electrode active material, and as a consequence there was concern that a battery that exhibits a high output and an inhibition of heat generation could not be obtained.